Genetic Process Engineering - Cellular Computing - page 70

is now less translational activity and therefore less input protein, φ A . Chapter 7

reports on experimental transfer curve results with different ribosome binding
sites for cI . Weaker RBSs caused an outward shift to the overall transfer curve
of the original cI /P(R) inverter.

INCREASING THE CISTRON COUNT The cistron count is increased by adding
copies of the coding sequence for the output protein downstream of the pro-
moter. As illustrated in Figure 4.19d, the increase in cistron count shifts L , the
translation stage mapping between

ψ A and φ A , upward. For any given level

of the mRNA input, φ A .

The curve values are scaled by a linear factor in the initial range of the transla-
tion before reaching metabolic constraints of the cell's translational machinary.
BioSPICEs simulations (Figure 4.22) demonstrate the overall outward shift of
the transfer curve of the cI /P(R) inverter as the cistron count increases from
one to five.

ψ A , there is now additional input protein repressor

ALTERING THE DEGRADATION RATE OF A PROTEIN The degradation rate of
proteins can be normally achieved by changing a few amino acid residues on
the C terminus [1, 11, 12]. For example, decreasing the half-life of the input
protein A causes L , the translation stage mapping between φ A , to shift

downward. Because of the increase in the protein degradation rate, for any given
level of the input mRNA

ψ A and

φ A , present.
Another possibility for increasing the degradation rate is to choose bacterial
strains that have high concentrations of effective proteases [16].

ψ A , there is now less repressor protein,

Different Cistron Counts for Modified Inverter

5

cistrons = 1
cistrons = 2
cistrons = 3
cistrons = 4
cistrons = 5
x=y

4

3

2

1

0

0 1 2 3 4 5

psi(A) = input signal

Figure 4.22 The effects of increasing the cistron count per gene (i.e., the number of
structural genes per operator region). In this case, the original inverter mechanism
has been modified to 100 × less binding affinity of the repressor and 40 × less binding
affinity of the promoter.